The invention is directed to a telecommunication network, and in particular to a return-to-zero optical modulator with configurable pulse width.
A typical 10 Gbps system uses non-return-to-zero (NRZ) coding formats to create a 10-Gbps signal, most often using a DFB laser and an external modulator to encode the signal.
Return-to-zero (RZ) encoding has lately emerged for ultra-long-haul 10 Gbps (for example in submarine transmission systems) and long-haul 40 Gbps because the RZ encoded signal has a higher peak power, providing a high signal-to-noise ratio and a corresponding low bit error rate. RZ also offers good immunity to the effects of polarization-mode dispersion (PMD) and polarization dependent loss (PDL), and can benefit from fiber nonlinear effects such as self phase modulation. Because symbols are isolated from one another, RZ pulses can take advantage of the soliton effect that arises from the interaction between fiber dispersion and self-phase modulation.
On the other hand, NRZ signals require a single external modulator, while RZ signals typically require two modulators in cascade, one to modulate the data and one to generate the RZ pulse shape, adding cost and complexity. In some cases, a third MZ is used to impart phase modulation (frequency chirp) on the RZ pulses. With this conventional approach, the RZ pulse generator is driven with a sinusoid at the line rate or one-half the line rate, depending on the bias point of the MZ, and the data modulator is driven with the NRZ modulated electrical signal. The resulting RZ pulse width is generally constrained to a full-width, half-maximum (FWHM) of 33%, 50% or 67% of a bit period when the MZ is biased at, respectively, maximum transmittance, one-half maximum transmittance, and minimum transmittance.
Having to use a second MZ device to generate RZ pulses, not only requires additional space in the transmitter terminal, but also adds cost to the terminal. In addition, the constraint on RZ pulse width limits a designer""s ability to optimize a system based on RZ pulse width. This cost is most important in DWDM systems, where each channel has its own transmitter, and the number of channels is currently up to 160.
It is an object of the invention to provide an optical modulator for optical communication of RZ encoded signals, which alleviates totally or in part the drawbacks of the prior art network architectures.
Accordingly, the invention provides for a return-to-zero (RZ) modulator for an optical transmitter, comprising a Mach-Zehnder device optically coupled with a light source for modulating a continuous wave generated by the light source with a modified drive signal; and a drive circuit for modulating the Mach-Zehnder device to generate an optical RZ pulse signal with adjustable width.
The invention also provides a method of generating an optical RZ pulse signal comprising: converting a non-return to zero (NRZ) signal to a modified drive signal, and modulating a continuous wave from a light source with the modified drive signal to generate an optical RZ pulse signal.
The invention provides for a less expensive optical terminal than the traditional solutions, due to the use of a single M-Z device. Additional cost savings are obtained since one bias circuit is necessary, as opposed to two such circuits when two M-Z devices are used.
Still another advantage of the invention is that it allows adjusting the width of the pulses in the optical signal.